Among many different types of electrical illuminating devices, Light Emitting Diode (LED) is becoming a popular light source increasing the utility of LEDs for many purposes including illumination. Light emitting diodes producing different colors, such as, red, blue and green LEDs are available. Combinations of these primary colors can produce almost any color enhancing LED use for many decorative lighting applications and illumination. A light emitting diode, being of small size, also has the potential to produce small size illumination apparatus, particularly with special power drivers to efficiently utilize them.
LEDs are well suited for implementing a color pixel in a digital image display by combining several LEDs to generate a range of desired colors at the pixel. In order to drive a color pixel consisting of three light emitting diodes each with one of the primary colors, typically requires three separate power supplies producing different voltage. Controlling these three power supplies separately enables the three LEDs to produce a desired color with a desired brightness. Most LEDs work at low voltages, typically 1.5V to 4 volt. Since red, blue and green LEDs all have different turn on or forward voltages, each of the power supplies must produce current at different voltages. Moreover, often a number of LEDs are connected in parallel in order to increase the brightness, thus requiring the power supply to provide a high enough current to drive the parallel LEDs.
A drawback of low-voltage high current power supplies is their low efficiency. This is because most switching power is supplied across an output diode having a forward voltage comparable to that of the intended LED load. Thus, voltage produced is shared between this diode and the LED and brings the efficiency down to nearly 50 percent with the high current producing high resistive losses.
A known method for avoiding the need for low-voltage power supply connects a number of LEDs in series so that the driving voltage is the sum of the voltage of each LED in connected in series. However, this arrangement reduces reliability because the failure of any one of the LEDs in the series arrangement results in the failure of the whole arrangement.
Moreover, it is desirable to have a single power supply rather than three separate ones for the three primary colors. However, as indicated above, LEDs corresponding to the three primary colors correspond to different forward voltage drops. Typically, a linear driver in placed in series with LED of each color while the series connection is connected to a single constant voltage power source. The driver takes up the voltage difference between the power source and the LED. However, this method is exhibits great power dissipation and low efficiency. The efficiency of this method is only around 50 percent as the voltage drop across the driver is often comparable to the forward voltage of the LED. An arrangement with such low efficiency produces significant heat resulting in the need for a heat sink increasing product size while reducing reliability.